Certain optical devices incorporate optical input and output fibers optically coupled within the device by one or more lenses and other optical elements, e.g., filters, that is, wavelength selective thin film filter elements, etc. The lenses are generally operative to collimate light emitted from an input or launch fiber along the optic axis, i.e., the optic axis of the lens (or to focus light passed into a receiving or output fiber). Examples of such devices include add/drop optical switches, interleavers and other multiplexing/demultiplexing devices (such devices being collectively referred to here in some instances as multiplexing devices or multiplexers), dispersion compensators, isolators, circulators, etc. Various lens types are known for use in such optical devices. GRIN lenses used in optical telecommunication systems, for example, are generally cylindrical in shape and about 1.8 mm in diameter and 4.8 mm long. Ball lenses for such devices are spherical and similarly sized.
The fiber optic industry employs lenses in such devices having a small focal length in order to reduce the size of the expanded (or collimated) beam. The standard lens employed by the fiber optic industry in such optical devices, especially in devices for telecommunication fiber optics, has a focal length of 2 mm or less. The short focal length lenses enable the use of small filter elements to receive the expanded beam (i.e., to reflect or pass all or a portion of the expanded beam). Such filters contribute substantial cost to such devices, and larger size filters would be substantially more costly and would be expected to result in substantially higher overall cost for the optical device.
The end of an optical fiber is positioned at the focal length of the lens. The signal beam launched out of the end of the fiber or received into it increases in size due to dispersion or divergence as it travels between the end of the launch fiber and the lens. The small focal length lenses used in optical devices result in correspondingly small distances between the fiber and the lens and, therefore, result in correspondingly low beam dispersion or divergence between the fiber and the lens. Advantageously, therefore, the expanded (or collimated) beam in the optical device has a small diameter. The small diameter expanded beam is passed between the lens and an associated filter, e.g., a selectively transparent interference filter or other components of the optical device. In optical devices employing selectively transparent interference filters, e.g., for isolating or adding one or more wavelength bands or channels of a multi-channel optical signal, as much as possible of the optical signal emitted from the launch fiber of the optical device through the lens to the filter should be received by the face of the filter in order to have good optical performance properties, such as low loss, etc. The same optical consideration applies to expanded beam signals passed from a lens into a receiving fiber. Thus, as noted above, the short focal length lenses employed in such optical devices result in a small diameter expanded beam within the device and, so, enables the use of small, economical filter elements to receive all or substantially all of the expanded beam.
Improved optical devices are required by the fiber optic industry, especially for use in fiber optic telecommunication systems. Improved devices are required, for example, to fully meet the substantial interest in reducing the wavelength spacing between adjacent channels in multiplexed signal carried by fiber optic lines and processed by optical devices, such as multiplexers, dispersion compensators, isolators, circulators, etc., e.g., to 50 GHz spacing or even 25 GHz spacing, 12.5 GHz spacing or less.